Any number of transactions can hold shared locks on an item, but if any transaction holds an exclusive on the item no other transaction may hold any lock on the item.
If a lock cannot be granted, the requesting transaction is made to wait till all incompatible locks held by other transactions have been released. The lock is then granted.
Database System Concepts 3rd Edition
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Lock-Based Protocols (Cont.)
Lock-compatibility matrix
A transaction may be granted a lock on an item if the requested lock is compatible with locks already held on the item by other transactions
A locking protocol is a set of rules followed by all transactions while requesting and releasing locks. Locking protocols restrict the set of possible schedules.
Basically, how to generate a correct (serializable) schedule?
Database System Concepts 3rd Edition
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©Silberschatz, Korth and Sudarshan, Bo Zhou
Pitfalls of Lock-Based Protocols
Database System Concepts 3rd Edition
16.1
©Silberschatz, Korth and Sudarshan, Bo Zhou
Lock-Based Protocols
A lock is a mechanism to control concurrent access to a data item Data items can be locked in two modes : 1. exclusive (X) mode. Data item can be both read as well as
written. X-lock is requested using lock-X instruction. 2. shared (S) mode. Data item can only be read. S-lock is
requested using lock-S instruction. Lock requests are made to concurrency-control manager. Transaction can proceed only after request is granted.
Database System Concepts 3rd Edition
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©Silberschatz, Korth and Sudarshan, Bo Zhou
Lock-Based Protocols (Cont.)
Example of a transaction performing locking:
T2: lock-S(A); read (A); unlock(A); lock-S(B); read (B); unlock(B); display(A+B)
Locking as above is not sufficient to guarantee serializability — if A and B get updated in-between the read of A and B, the displayed sum would be wrong.
and its locks released.
Database System Concepts 3rd Edition
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©Silberschatz, Korth and Sudarshan, Bo Zhou
Pitfalls of Lock-Based Protocols (Cont.)
The potential for deadlock exists in most locking protocols. Deadlocks are a necessary evil. Starvation is also possible if concurrency control manager is badly designed. For example:
Chapter 16: Concurrency Control
Lock-Based Protocols Multiple Granularity Deadlock Handling Insert and Delete Operations Concurrency in Index Structures
Consider the partial schedule
Neither T3 nor T4 can make progress — executing lock-S(B) causes T4 to wait for T3 to release its lock on B, while executing lock-X(A) causes T3 to wait for T4 to release its lock on A. Such a situation is called aБайду номын сангаасdeadlock. To handle a deadlock one of T3 or T4 must be rolled back